Scientists at the Max Planck Institute of Quantum Optics, the LMU Munich and the University of Basel have now demonstrated a new technique for the imaging of microwave magnetic fields. As microwave field sensors, they use small clouds of ultracold atoms that had been laser-cooled to a temperature of a few millionths of a degree above absolute zero. At these temperatures, the atoms obey the laws of quantum physics. Their quantum state is very sensitive to externally applied electro- magnetic fields, which makes them ideal sensors. For the measurement, the atoms are positioned at the desired location above the microwave circuit with the help of static magnetic fields, and subse- quently the microwave field is turned on.

“The internal state of the atoms changes if a micro- wave field is applied,” Pascal Böhi explains, who co-developed the technique as part of this doctoral thesis. “We can image this change of internal state with a CCD camera with high spatial resolution. The stronger the microwave field at a given position, the faster the rate of change we observe.” A unique fea- ture of the new method is that it does not require